The present invention relates to a method of continuous production on a line of a metal strip such as a steel rolled metal sheet, from a heat formed strip.
It is generally known that producing metal products first requires the production of a coarse product by ingot casting or by continuous casting, a heat forging and/or heat rolling treatment and a cold treatment comprising several steps depending upon the metal nature, for example ferritic or austenitic steel, and upon the quality of the product to be produced.
Usually, the heat formed product is submitted, successively, to a de-scaling treatment for removing scales, to a cold rolling process until a desired thickness has been obtained, and, finally, to finishing treatments.
The cold rolling process is usually carried out in several successive passes, either in two opposite directions on a reversible train, or on several roll stands operating as a tandem.
It is known that, in a rolling mill, the product is driven between two working rolls the spacing of which is less than the rough thickness of the upstream product. A metal flow occurs which is friction driven in the gripping gap between the rolls up to an outlet section the thickness of which substantially corresponds to the spacing between the working rolls, with a progressive speed increase which corresponds to the metal preservation.
During a rolling process, the working rolls tend to be spaced apart one from another and the clearance between the opposite generators must therefore be maintained by applying, between the rolls, a clamping effort, often so-called rolling power.
The rolling power to be exerted in order to obtain a certain thickness reduction rate depends of course upon the diameter of the working rolls and upon the metal composition: common poorly alloyed low carbon steel, stainless steel, alloyed steel, as well as upon the features thereof, more particularly the yield point.
Under the action of the rolling power being applied between their ends, the rolls tend to be bent. Moreover, deformations and collapses of the different elements of the roll stand occur, in particular the columns supporting the clamping means.
Several means are available in a roll stand to compensate for the deformation of the rolls and the stand, in order to keep the thickness regularity and correct the flatness defects by modifying the stress distribution along the backing generators of the working rolls.
In normal operation, these correcting means make it possible to hold the strip qualities on the largest length of the spool, often referred to as xe2x80x9cstrip bodyxe2x80x9d. However, at the start of a new spool unwinding, the upstream end or xe2x80x9cheadxe2x80x9d thereof must be inserted into the installation, on a sufficient length so as to allow for strip driving. Therefore, the new spool head may be welded onto the downstream head, or xe2x80x9ctailxe2x80x9d, of the preceding spool. Similarly, at the device output, when the spool being wound has reached its maximum size, the strip has to be sheared so as to release the spool and secure the upstream end of the subsequent strip portion on the winding mandrel.
Reducing the unwinding speed cannot be therefore avoided, more particularly in the cold rolling stands,.such a speed even becoming nil if a single winding core is being used.
As a result, the strip qualities cannot be usually held on the spool head and tail, which must be eliminated.
Moreover, it is necessary, before cold rolling, to eliminate, as much as possible, the oxidation produced scales due to the previously undergone treatments and, to that effect, several known methods can be used, for example shot-blasting or chemical de-scaling, etc.
In the past, the various cold treatments were continuously carried out in separate sections, the product being wound into a spool at the end of each section in order to be transferred to the following section. Nevertheless, these batch processes have the disadvantage that they multiply the spool winding and unwinding processes and that they require intermediary storage steps leading to high costs, because of the indispensable handling devices and the necessary personnel.
Manufacturing processes have been developed for a few years, allowing to eliminate the spool winding at least for some intermediary steps, the spool successively running in at least two treatment sections located on a continuous line. Accumulators are mounted between the successive sections in order to make it possible for them to work at different instantaneous speeds. The spool running step may thereby be slowed down or even stopped in a section, for example in the case of a mishap or when changing spool, whereas the other sections keep on working.
In particular, coupling the de-scaling process with the cold rolling process makes it possible to reduce substantially the above-mentioned disadvantages.
On such a continuous line assembly, rolling is performed in a tandem rolling mill having normally four to five roll stands.
Such an assembly is described, for example, in xe2x80x9cDxc3xa9capage-tandem couplxc3xa9 de Sainte Agathe à Sollac-Florangexe2x80x9d, published in xe2x80x9cLa Revue de Mxc3xa9tallurgiexe2x80x9d, March 1998.
Until now, the hot forming and treatment methods allowed to produce heat strip spools with a relatively high thickness. In order to obtain the usually required thickness, the thickness reduction rate to be performed was therefore large, generally in the range of 70% to 80% and up to 90% for some steel grades.
The rolling mills adapted to develop the necessary power are very expensive and usually the coupled line assemblies are therefore optimised favouring the tandem rolling mill working that is the bottleneck of the assembly.
More particularly, as just described, the rolling mill must at least slow down at the spool change and, if the running speed decreases, the friction coefficient increases, so does the rolling power to be applied between the working rolls to obtain the required thickness reduction. Moreover, the deformations and collapses within the roll stand increase as well. It has thus been observed that there is a boundary under which rolling cannot be performed with a thickness adjustment.
Usually, one therefore attempts to maintain a relatively high speed, in the order of 300 m/min, for example, so as not to enter a field where a slow speed would make it impossible to hold the thickness quality.
As the strip shearing cannot be avoided for the spool change, at the assembly outlet, it is necessary to minimise, as much as possible, the period of time needed for this operation. To that effect, strip shearing is usually carried out at a relatively high speed using a so-called xe2x80x9cflyingxe2x80x9d shear and two successively actuated winding mandrels are used so that, during the time needed for unloading the spool wound on the first mandrel, the strip can be engaged and starts to wind on the second mandrel, immediately after the shearing step.
Under these conditions, the strip quality is only affected for a low value, 1% or a few %, and on a short distance, which may be limited to a few meters. Thus, the yield per 1000 kg of the assembly remains acceptable. Moreover, the external spool face serving as a wrapping thereof can anyway be more or less deteriorated when being handled and is therefore sacrificed.
However, to optimise the working of the rolling mill, it should be adapted to determine product types and the other sections of the coupled line, more particularly the de-scaling and finishing assemblies, must be provided accordingly.
That is why, until now, the strip producing assemblies on a continuous line were essentially provided for products having a very large capacity, in the order of 1 to 2 million tons a year, for example automotive bodywork sheets.
For special products requiring a more limited capacity, it would be more profitable to perform the rolling operation in successive passes, in one direction and in the opposite direction, on a reversible roll stand.
On the other hand, for hard or very thin products, for example in stainless steel, it is usually preferred to use a small roll rolling mill of the SENDZIMIR type.
It has been also contemplated, for the production of stainless steel strips, to manufacture coupled assemblies, on a continuous line, but, in this case, the various treatments should be performed under very accurate conditions so that the product has the desired properties.
In all cases, paradoxically, extremely expensive assemblies are therefore available, the working conditions of which are quite strict.
But technology constantly goes forward and, for example, the manufacturing and heat treatment methods have recently undergone large developments that should be considered.
Additionally, the clients"" needs are changing and it can therefore be boring, even for producing conventional low carbon poorly alloyed steel sheets, to use very expensive assemblies being simultaneously specialized in a particular production type.
The object of this invention is therefore to overcome such inconvenience with a new production method, on a continuous line, for a rolled sheet, allowing to adapt very flexibly to a change in the production conditions, while holding the possibility in any cases to solve all the above-mentioned problems. In particular, the invention makes it possible to keep the thickness regularity, and in a general way, the sheet quality on a strip length at least of the same order as in the most performing present assemblies and even practically on the whole spool.
The invention therefore generally relates to the production of a metal strip from a heat produced product, by running the strip, on a continuous treatment line having, in one running direction of the strip, an inlet section, a de-scaling section for eliminating the calamine, an accumulating section, cold rolling means and an outlet section having a shearing element and winding means.
According to the invention, in order to adapt to a change in the operating constraints, more particularly a slackening of speed for the spool change, the rolling speed is caused to vary on a wide range from less than 1 m/min to more than 1000 m/min and the cold rolling operation is performed in maximum three passes between the working rolls the diameter of which is defined so that, on the whole speed variation range, the rolling power needed to maintain the thickness reduction rate at each pass remains compatible with the thickness adjustment and product flatness possibilities, taking the characteristics thereof into account.
The invention may exhibit advantages for all steel types but, in practice, it essentially applies to the production of conventional low carbon and/or poorly alloyed and/or low yield strength steel sheets.
In a particularly advantageous way, for cold rolling, working rolls are provided the diameter of which does not exceed 200 mm.
In practice, the cylinder diameter will be determined so that the rolling power needed at the lowest speeds does not exceed twice the rolling power at the highest speeds.
The invention specially relates to the production of sheets made of soft or poorly alloyed steel for which the rolling operation will be advantageously limited to two passes, taking the yield strength of the steel and the reduction rate to be obtained into account.
According to another preferred feature, in order to lubricate and to cool roll stands, an oil-in-water emulsion is used, the saponification index of which does not exceed 50.
The invention also relates to a continuous line assembly for industrial production of a steel sheet strip for carrying out the method, comprising successively:
a primary de-scaling section for eliminating calamine;
a continuous cold rolling section,
an outlet section comprising a strip-shearing element for cutting and spooling and a strip-winding device.
In a particularly advantageous embodiment, the invention relates to the production of sheets of soft, low carbon and/or poorly alloyed steel and relates also to an assembly wherein the rolling section comprises three roll stands at most, the shearing element is of stationary type and the winding device comprises only one winding mandrel, such an assembly being particularly cost effective.
Preferably, the de-scaling section may be of the chemical or electrochemical type and may then comprise a xe2x80x9cbreak-oxidexe2x80x9d tensile planing device.
Advantageously, depending upon the applications and the steel shades, the de-scaling section may also comprise a shot blasting and/or an abrasion device.
According to an essential feature of the invention, the cold rolling process is done in roll stands provided with small diameter working rolls. To this effect, it will be particularly advantageous to use multi-roll stands with intermediary rolls arranged in clusters of the xe2x80x9ccluster millxe2x80x9d type or roll stands the working rolls of which are associated to side backing rolls, in particular of the xe2x80x9cZ-highxe2x80x9d type.
The roll stands will be advantageously provided with a device for checking the strip flatness. For example, at least one backing roll will be of the deformable rotating jacket type.
In a particularly advantageous way, on a continuous line for implementing the invention, the cutting out point may be located at such a distance from the winding core that the strip length being developed up to the gripping gap of the last roll stand of the rolling mill is higher than 20 meters, the thickness regularity remaining secured in the portion constituting the strip body.
This way it is possible to provide, between the rolling mill and the shear, some secondary elements such as a tensioning device and/or oiling means for the strip.
But the method according to the invention may also be applied to lines the cutting out point of which is located at such a distance that the strip length being developed up to the gripping gap of the last roll stand of the rolling mill is less than 20 meters, the thickness tolerance being tighter in the portion constituting the strip ends.
According to another very advantageous feature, a continuous line assembly according to the invention may also be used to treat heat produced strips which only require a low or no thickness reduction, the cold rolling last pass simply performing a finishing treatment of the xe2x80x9cskin-passxe2x80x9d type. The rolling stands will then advantageously be quarto rolling stands of the xe2x80x9cZ-highxe2x80x9d type wherein each assembly formed with a working roll, an intermediary roll and of the side backing rolls, is an insert that can easily be replaced by a working roll having a large diameter so as to make a skin-pass working roll stand of the quarto type.
In that case, the large diameter last rolling stand may advantageously be used to impart to the sheet quite a significant roughness, for example of at least 0.4 micrometers so as to enhance the adherence of a protective coating.